The present invention relates to dual check valves of the type where each check valve is located in a single fluid passage and where each valve member is spring biased to a closed position whereby opening of the valves will occur in response to a predetermined increase in fluid pressure at the inlet to the valve housing.
In a number of fluid handling systems, dual type check valves have frequently been used chiefly as safety devices to guard against back-flow in the fluid systems. Further, in the past, dual check valves have been configured so that the resilient force urging the downstream valve towards its closed position would be utilized to assist in maintaining the upstream located valve member in its closed position. Further, fluid pressure acting on the upstream valve member would, in many structures, be transmitted through the upstream valve member to the downstream valve member thus often negating the benefit of having dual check valves in the fluid passage.
Recognizing these disadvantages, a new ASSE standard now requires that each of the check valves in a dual system have no common parts except for the body housing and that there be no contact between any moving parts of either of the check valves in the course of their operation. Thus, the failure of either one of the check valves will have no effect on the operation of the other check valve.
As a consequence of this standard change, it has been necessary to reequip a number of dual check valve assemblies in order to assure compliance with the standard particularly where certification of the fluid handling system is required such as by a government agency.
One of the principle difficulties confronting valve designers in these circumstances is the provision of a dual check valve assembly with independently operating check valves that can be installed within the design constraints of an already existing fluid handling system to avoid the expense of realigning high pressure conduits to accommodate new valve housings of different or increased dimensions.